KABUL (Reuters) - Tens of thousands of
volunteers fanned out across Afghanistan this week, braving
deteriorating security and distrusting parents to administer two chilled
drops of the oral polio vaccine each to millions of children.

Keeping the highly infectious polio disease in check in any country
is a daunting task. But in a nation where Taliban militants are fast
gaining ground against government forces, it's also a dangerous one.

Afghanistan is one of only three nations where the polio virus is
still endemic, along with Pakistan and Nigeria. For a nation at war,
its anti-polio campaign has had remarkable success, bringing the
number of cases down from 63 in 1999 to just 14 in 2013. Only eight
new cases have been confirmed so far this year, compared to 108 in
Pakistan.

But as fighting between Afghan forces and militants intensifies
ahead of the withdrawal of most foreign troops this year, health
workers risk losing precious access to the places - and children -
they need to keep tabs on.

This week, in some restive areas of the east and southeast, health
workers had yet to go door to door to deliver the vaccine, said Dr.
Mohammad Wasim Sajad, a training officer in the Ministry of Public
Health in Kabul.

"People are not willing to go out," he said, adding that
negotiations with local groups to allow vaccinators to do their work
safely were under way.

Vaccination is the only known way to prevent polio, an infectious
disease that attacks the nervous system mainly in children under
five and can lead to permanent paralysis and death. It has no cure.

"We don't see a big problem now, but if (major) fighting continues
long term, then access will be difficult," said Abdul Majeed Siddiqi,
the head of mission in Afghanistan for HealthNet TPO, an NGO that
advises the Afghan government on polio.

People's attitudes toward the vaccine are another challenge.

In a dusty hillside neighborhood of Bagh Qazi, Freshta Faizi, a
volunteer, trudged from house to house on Monday, asking residents
if their children had been given their drops.

"Sometimes they just say no and shut the door," said Faizi. "They
say it won't make any difference. I've had people tell me the
vaccine is just American urine."

In August, Human Rights Watch reported that in parts of the southern
province of Helmand, the Taliban had stopped health officials from
sending out mobile vaccination teams.

That was an alarming development, because - unlike some militant
factions in Pakistan, which have targeted and killed anti-polio
campaigners - the Afghan Taliban have pledged support for
vaccination.

A Taliban spokesman said the group had concerns that some polio
vaccinators in Helmand were promoting government policy, not health.
However, he told Reuters by phone that after the health ministry had
organized talks on the issue, the group was satisfied. "There are no
more problems," he said.

Though a win for the campaign, the scare underscores the fragility
of Afghanistan's gains. Negotiations on allowing vaccinators to move
freely throughout the country were conducted locally and on a
"case-by-case" basis, said Sajad.

Ten years ago, Afghanistan was tantalizingly close to halting the
circulation of the virus within its borders: only four cases were
confirmed in 2004, according to the government.

But as security deteriorated, health workers couldn't travel to
dangerous areas and were unable to make sure children were getting
the vaccine. By 2012, the number of polio cases in Afghanistan rose
to 37.

AN UNEXPECTED OPPORTUNITY

Complicating matters, this summer over 150,000 refugees from North
Waziristan, a tribal region of Pakistan where leaders banned the
polio vaccine in 2012, poured over the border into Afghanistan,
seeking refuge from a military offensive against insurgents.

The influx from an area that has generated most of Pakistan's polio
cases this year immediately raised alarm.

Most of Afghanistan's new cases of polio this year are genetically
linked to Pakistan, according to HealthNet TPO. But the government
says that, so far, only one case has been identified as having
coming from a North Waziristan refugee.

The flood of refugees yielded an unexpected opportunity, however:
over 400,000 vaccine doses were given out in Pakistan along the
routes residents used to flee their homes, according to the Global
Polio Eradication Initiative.

That's good news for the global fight to exterminate polio and
particularly for Afghanistan, where thousands of residents move
across the Pakistan border every day.

"If there is control in Pakistan, there will be control in
Afghanistan," said Siddiqi. "If there is no control in Pakistan, the
problems in Afghanistan will continue."

(Additional reporting by Hamid Shalizi in Kabul; Writing by Krista
Mahr; Editing by John Chalmers and Nick Macfie)